In a hybrid vehicle or an electric vehicle, an engine is limitedly used or not used. Thus, engine cooling water required for heating a vehicle room may be insufficient or may not be obtained.
Accordingly, an air conditioning system for a hybrid vehicle or an electric vehicle (hereinafter generally referred to as a “motor vehicle”) employs a heating device capable of heating a vehicle room without engine cooling water. As one example, there is available a heating device that makes use of a positive temperature coefficient (PTC) heater.
As shown in FIG. 1, the heating device using the PTC heater has a structure in which a PTC heater 5 is installed in a hot air path 3 of an air conditioner case 1. The PTC heater 5 is a high-voltage heater, the heat generation amount of which is adjusted by the control of a pulse width modulation (PWM) duty ratio.
In the PTC heater 5, a specific target heating temperature is set. If a PWM duty ratio corresponding to the target heating temperature is applied to the PTC heater 5, the PTC heater 5 generates heat in conformity with the PWM duty ratio thus applied. Thus, the PTC heater 5 heats the air blown into a vehicle room while being controlled in conformity with the target heating temperature. In this way, the PTC heater 5 heats the vehicle room.
In the conventional air conditioning system mentioned above, if the opening degree of the hot air path 3 is sharply reduced in the course of operating (turning on) the PTC heater 5, the volume of the air passing through the PTC heater 5 decreases. In this case, the heat exchange amount of the PTC heater 5 is reduced and the heating temperature of the PTC heater 5 is rapidly increased. This poses a problem in that the PTC heater 5 is overheated.
Particularly, when the temperature door 7 is moved to a maximum cooling position X to minimize the opening degree of the hot air path 3, the volume of the air passing through the PTC heater 5 is significantly reduced. In this case, the heating temperature of the PTC heater 5 is sharply increased. Thus, the PTC heater 5 may be overheated. This may pose a problem in that the PTC heater 5 is thermally deformed or damaged and the air conditioner case 1 existing around the PTC heater 5 is also thermally deformed or damaged.
In view of the above problem, there has been proposed a technique which prevents the overheating of the PTC heater 5 by controlling the temperature door 7 so as not to be completely closed and allowing a minimum amount of air to be blown toward the PTC heater 5. In this technique, the temperature door 7 is controlled such that a minimum opening degree of the hot air path 3 is secured even if the temperature door 7 is moved to the maximum cooling position X. This enables a minimum amount of air to be blown toward the PTC heater 5, thereby preventing the overheating of the PTC heater 5.
However, the conventional technique mentioned above is configured such that the hot air path 3 is opened at a minimum level regardless of the on/off condition of the PTC heater 5. Thus, there may be a case where the hot air path 3 is unnecessarily opened even when the PTC heater 5 is turned off. This may pose a problem in that the air coming from an evaporator 9 is unnecessarily introduced into the hot air path 3.
The air coming from the evaporator 9 should unnecessarily pass through the PTC heater 5 installed in the hot air path 3. This may generate an unnecessary air resistance. As a result, the cooling and heating efficiency of the vehicle room is reduced.
In the conventional technique, a minimum opening degree of the hot air path 3 is secured in order to prevent the overheating of the PTC heater 5. Therefore, the air is introduced into the hot air path 3 even when the PTC heater 5 is turned off. In order to minimize the unnecessary air introduction into the hot air path 3 when the PTC heater 5 is turned off, it is necessary to limit the minimum opening degree of the hot air path 3.
In the case where the heating temperature of the PTC heater 5 is increased to such a level that the overheating probability of the PTC heater 5 is very high, for example, in the case where the target heating temperature of the PTC heater 5 is very high and the possibility of overheating of the PTC heater 5 is very high, the limited minimum opening degree of the hot air path 3 cannot cope with the overheating of the PTC heater 5.
Thus, even if a minimum air volume is secured to prevent the overheating of the PTC heater 5, it is impossible to prevent the overheating of the PTC heater 5.